Electromagnetic device for amplitude-modulation of high-frequency oscillations



y 4, 1953 w. J. VAN DE LINDT ,7

ELECTROMAGNETIC DEVICE FOR AMPLITUDE-MODULATION OF HIGH-FREQUENCY OSCILLATIONS Filed April 14, 1951 INVENTOR WILLEM JACOBUS VAN DE LINDT AGENT Patented July 14, 1953 ELECTROMAGNETIC DEVICE FOR AMPLI-- TUDE MODULATION OF HIGH FRE- .QUENCY OSCILLATIONS Willem-Jacobus van 'de Lindt, Eindhoven, Neth' erlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn as trustee Application April 14, 1951, Serial No. 221,024 In the Netherlands April 20, 1950 This invention relates to an electromagnetic device for producing amplitude-modulation of a high-frequency oscillation having a frequency of, for example, more than 100 mes/sec. ticular the invention relates to a device of the foregoing type employing a body of premagnet ized substantially non-conductive ferromagnetic material which produces a variable'damping of the high-frequency oscillation when excited by a modulating field. The term amplitude modulation is herein defined as including the control of amplitude of a wave as a function of a control quantity.

A device has previously been suggested in which a body of substantially non-conductive, ferromagnetic material, for example, a, ferrite (chemical formula Me(Fe2O4)t where Medesighates a metal or a combination of metals in which the sum of the va1encies=2n) is employed which is polarized by the action of a modulating field at right angles to the magnetic field strength In par.-

6 Claims. (Cl. 332-54) of the high-frequency oscillationin a manner such that the precession frequency (Larmor frequency) of the electrons of the ferromagnetic medium providing the magnetic properties, which frequency corresponds to said modulating and polarizing magnetic field, approximately coincides with the frequency of the high-frequency oscillation.

A device of the foregoing type has some important limitations. Thus, the polarizing field appears to require a constant component which saturates the ferromagnetic material to a large extent in order to dominate the disturbing effects of the anisotropic fields occurring Within the material, The value of this constant component of the field is furthermore proportional to the frequency of the high-frequency oscillation, so that for frequencies of, for example, morethan 100 mos/sec. a constant component of several thousands of amps/cm. is required. To produce such a field a large amount of directecurrent energy or a strong permanent magnet is required. Furthermore, it appears that modulation of the polarizing field not only results ina variation of the damping, i. e. of the imaginary permeability of the medium, but also in a material variation of the real permeability, so that, by placing the ferromagnetic medium, for example, in a cavity resonator excited by the high-frequency oscillation, the cavity resonator is materially detuned upon said modulation, so that the high-frequency oscillation is modulated not only in amplitude but also in a phase.

According to the invention, the ferromagnetic material'employed for this purpose is required to exhibit a specific resistance, measured at the frequency of the high-frequency oscillation, of more than 10- 9 cm. and furthermore the material should have a preferential direction of the partitions (Bloch partitions) between the Weiss domains createdby internal anisotropy of shape.

'Moreover, the direction of the magnetic field vector of the high-frequency oscillation forms an angle with this preferential direction. In addition, the pre-magnetizing fieldstrength' is such that the value of the saturation field strength of the ferromagnetic material is not exceeded so that its real permeability at the stated frequency generally varies slightly.

The invention is based on the discovery of the surprising effect that certain types of ferrite which are produced in a particular manner when arranged in a cavity resonator excited b'y'a highfrequency oscillation, exhibit an appreciable reduction in the damping of the cavity resonator with very slight pre-magnetisation. In general ferrites and their manner of preparation have been been fully described in U. S. Patents 2,452,-

529, 2,452,530 and 2,452,531 and U. S. applica- 1 high-frequencies concerned, exceeding 10 S2 cm.

The reason for this effect appears to be due to the fact that in conductive, ferromagnetic material the surfacecurrents (skin effect) prevent a high frequency alternating field from penetrating into the interior of the material, whereas in non-conductive ferrites the interior of the medium is also capable of exerting its influence upon the highfrequency alternating field since there are virtually no surface currents due to the poor conductivity of the latter materials. Thus, for example,

Mn-Zn ferrites having a large ferrous iron content, and having a specific resistance measured may be controlled during the process of manufacture of such ferrites by controlling the composition of the atmosphere in which the ferrite is manufactured.

Furthermore, it has been found that this effect occurs only at very high frequencies, viz. higher than the natural gyromagnetic resonance frewhich are manufactured by extrusion or, in general, those which exhibit a preferential direction of the partitions between the Weiss domains resulting from internal anisotropy of shape. This may be explained in that the preferential direction which the Bloch partitions exhibit as a result of extruding the body, appears to result in the ferrite showing an increase in damping at a high frequency of the alternating current field at right angles to this preferential direction, the Bloch partitions disappearing if the ferrite is pie-magnetized. It has been found from measurements that only a small pre-magnetizing field of several amps/cm. is required to vary the losses brought about by the ferrite, for example by a factor 10:1, so that in practice variation in amplitude (modulation depth) of the high-frequency oscillation is rendered possible, the tuning frequency of the cavity resonator then generally remaining substantially constant. The frequency of the pre-magnetizing field may be increased to several mos/sec.

The invention will now be explained more fully with reference to the drawing in which,

Fig. 1 shows a side view of the device;

Fig. 2 shows a plan view of the same device according to the invention;

Fig. 3 shows pictorially the action of the mag netic fields in a portion of the device, and

Fig. 4 shows a modulation characteristic curve of the device.

An electro-magnetic oscillation E1 is supplied through an input wave channel to a resonator 2, the circuit quality of which is varied with a modulating oscillation, so that a modulated oscillation E is produced in output wave channel 3. A ferrite rod 5, which is subject to the action of a modulating current i, which traverses a premagnetizing winding 5 surrounding the rod 5 is coupled to the cavity resonator. The oscillation produced in the cavity resonator 2 is such that the high-frequency magnetic field H is at right angles to the rod axis. A knob 1 enables it to be accurately tuned to the frequency of the oscillation Er.

According to the invention, the rod 5 is made of ferrite of the substantially non-conductive type, i. e. a ferrite having a specific resistance, measured at the stated high-frequencies, higher than 109 cm. It is, for example, made of NiZn ferrite having a low ferrous iron content, the specific resistances measured at high-frequency being about 9 cm.

It is furthermore necessary that the rod exhibit a preferential direction of the Bloch partitions produced by internal anistropy of shape and for this purpose it may, for example be made by the extrusion method in a known manner. The following, as it appears, is responsible for this requirement.

A rod manufactured by extrusion exhibits, as shown in Fig. 3, a considerable number of very small occlusions 9 occupied by a non-ferromagnetic phase, for example, air (for the sake of clarity the figure only shows two such occlusions on a greatly exaggerated scale), all of which will be elongated in the direction of the axis of the rod. The ferromagnetic material i. e. the material enclosing the occlusions 9, has an internal geometry which, viewed in the direction of the axis of the rod, distinctly differs from the geometry viewed at right angles to this axis, i. e. it has an anisotropy, geometric shape.

The Bloch partitions between neighbouring Weiss domains of the ferromagnetic material, which, in the absence of the occlusions 9, woul have adjusted themselves according to the bisector plane I0 of the directions I1 and In of the spontaneous magnetisation in these Weiss domains, will now rather tend to form the shortest connections [2 between neighbouring occlusions 9 and thus exhibit a preferential direction coinciding with the axis of the rod, in which event, however, magnetic charges are produced along this partition.

It is plausible that a high-frequency alternating field H, which invaria ly tends to dis place the Bloch partitions to such an extent that Weiss domains in which the magnetisation I has the direction of the magnetic field strength H, increase at the expense of Weiss ranges in which such is not the case, will have to supply an appreciably greater quantity of energy for displacing said partitions against the action of the mag netic charges, than if the occlusions 9 were absent or at least not elongated to such an extent.

Because of a premagnetizing field H0, which is produced by the current i flowing through the windings 6, the various Weiss domains with a direction of the spontaneous magnetization I corresponding to the field Ho will steadily grow, the Bloch partitions between the Weiss domains finally disappearing completely at the saturation field strength.

Consequently, in this case, the source of losses for the high-frequency alternating field H is rendered ineffective, the cavity resonator 2 having a high circuit quality and hence a high impedance, which results in the reflection of a great portion Er of the incoming oscillation E1. Accordingly, it is found, as shown in Fig. 4, that the amplitude of the high-frequency oscillation Eu has dropped considerably, for example, within a few amps/cm. By providing a tuning piston in wave guide 3 it may be ensured that the amplitude of the resulting oscillations, reflected by the cavity resonator and the piston is zero at a given value of the premagnetizing field H0, for example at a few amps./cm., the amplitude having its maximum value if the field Ho=0. With small fields Ho, the variation in the virtual permeability of the rod for high-frequency is in most cases so small that troublesome phase modulation of the high-frequency modulation does not occur.

Variation of the premagnetizing field H0 thus results in modulation of the output oscillation E0 in accordance with the modulation characteristic curve shown in Fig. 4. This curve is substantially straight through a great part, so that the process of modulating introduces only slight distortion, while furthermore the modulating field Ho may be raised to frequencies of the order of some mos/sec. without the amplitude of the modulated oscillation varying to any appreciable extent with the frequency of the modulating oscillation. If the direction of the premagnetizing field H0 is reversed, a similar characteristic curve is found (shown by dotted line in Fig. 4).

The required premagnetizing field is, of course, reduced if the ferrite rod is madelonger, since in this case the demagnetizing action of the end Example I A mixture consisting of 75 gs. (1 mol.) of nickel oxide and l60gs. (1 moll) of ferri-oxide, after alcohol being added, is ground in a ball mill for 16 hours. After filtration and drying at 110 C. the mixture is heated at 1000 C. for2 hours, then after addition of alcohol is ground again in the ball mill for 16 hours and subsequently filtered and dried.

10 gs. of this mixture, together with 2 ccs. of water and 300 mgs. of Electrocol (a partly hydrolised potato-starch product of Messrs. Scholten at Groningen) are treated in a mortar for minutes and subsequently kneaded by hand for 5 minutes, if desired whilst adding some further drops of water, to form an easily kneadable mass. The mass is extruded in anjextrusion mould to form a rod of 1.57 mms. in diameter, from which a D-shaped ring is formed by hand, having a straight piece of about 40 mms. The

ring is dried in air and subsequently heated at 1230 C. in an atmosphere of oxygen for 2 hours.

The measurements carried out on this rin yielded the following results: I

(The term half-value width is to be understood to mean the width of the resonance characteristic curve of the cavity resonator where the impedance has decreased to half the maximum value.)

Working frequency, 9260 mos/sec. Initial permeability, 17.

Specific resistance at working frequency, 10

resonance frequency=l30 mcst/ Example II A mixture consisting of 750 gs. (10 mol.) of nickel oxide, 810 gs. (10 mol.) of zinc oxide and 3200 gs. mol.) of ferri-oxide, after water being added, is ground in a ball mill for 16 hours. After filtration and drying at 110 C., the mixture isv heated at 1000 C. for 2 hours and subsequently, after addition of water, ground in the ball mill for 16 hours, filtered and then dried.

A quantity of 10 gs. of this mixture is treated in the manner stated in Example I, to form a plastic mass, from which a rod of 1.62 mms. in diameter' and 104 mms. of length is extruded and heated at 1230 C;, in air for 2hours.

' .Theresults of measurement .carried out in this rod were as follows: I

Working frequency=9300 mos/sec. Initial permeability, ,1i0=230.

Specific resistance at working frequency, 10

Gyromagnetic resonance frequency: 15 mos/sec.

Cavity resonator box-like, diameter 56 mms.,

height 15 mms.

Oscillation curve E020.

Half-value width cavity resonator without rod:

6 mos/sec.

Contribution half-value width by rod at H0=0,

22.6 mos/sec.

Contribution half-value width by rod at H0=4= amps./cm., 9.4 mos/sec.

Detuning cavity resonator at H0=0, 2 mcsL/sec.

Detuning cavity resonator at H0=4' amps./cm.,

6mcs./sec. q 1 v Example III A mixture consisting of 60.4 gs. of raw manganese carbonate (corresponding to 0.5 mol. of MnO) and 80.1 gs. (0.5 mol) of ferrieoxide, after alcohol being added, is ground in a ballmill for 8 hours, subsequently dried and then heated at 950 C. in air for 2 hours. After addition of alcohol, the grinding process in the ball mill is continued for 4 hours, followed by drying at C. 10 gs. of this mixture are treated in the manner indicated in Example I, to form a plastic mass, from which a rod of 2.2 mms. in diameter and 40 mms. of length is manufactured bythe extrusion process. The rod is dried in air, slowly heated to 300 C. and; then evenly heated to -l250 C. and heated at this temperature in an atmosphere of substantially pure nitrogen for 2 hours.

Measurements carried out on this rod yielded the following results:

without rod:

Detuning cavity resonator at H0=O, '7 mos/sec.

Detuning cavity resonator at H0=30 amps./cm., 70 mos/sec. r 0

Similar measurements carried out in a cavity resonator for a working frequency=3100 mcs./

see. with correspondingly thicker rods showed anapproximately equal variation and the half-value width of the cavity resonator and a detuning about 5 times smaller. Similar results were found at a working frequency of 310 rncs./sec.; at a working frequency=22,000 mcs./sec., the variation in' half-value width and hence of the circuit quality with the premagnetizing field started to decrease.

The embodiment shown in Fig. 2 only serves as an example of a device according to the in-- vention. The cavity resonator may alternatively be realized in numerous other ways.

While the invention has thus been described with specific examples and embodiments, other variations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the invention.

What I claim is:

l. A device for amplitude modulating a cyclically recurring wave, comprising a, body of substantially non-conductive ferromagnetic material, means to generate a wave of given frequency, higher than the natural gyromagnetic resonance frequency of said ferromagnetic material, means to couple said wave generating means to said body, said body having a specific resistance exceeding ohms/cm. at said given frequency, and having a given preferential direction of Bloch partitions produced by internal anisotropy of shape said preferential direction forming an angle with the magnetic field vector of the wave, and means to premagnetize said body below a field strength of the order of magnitude of the saturation field strength of the ferromagnetic material.

2. A device for amplitude modulating a cycli cally recurring wave, comprising a body of substantially non-conductive, ferromagnetic material, means to generate a wave of given frequency higher than the natural gyromagnetic resonance frequency of said ferromagnetic material, means to couple said wave generating means to said body, said body having a specific resistance exceeding 1O ohms/cm. at said given frequency, and having a given preferential direction of Bloch partitions produced by internal anisotropy of shape substantially perpendicular to the magnetic field vector of the wave, and means to premagnetize said body below a field strength of the order of magnitude of the saturation field strength of the ferromagnetic material.

3. A device for amplitude modulating a cyclically recurring wave, comprising a substantially rod-like body of substantially non-conductive, ferromagnetic material, means to generate a wave of given frequency higher than the natural gyromagnetic resonance frequency of said ferromagnetic material, means to couple said wave generating means to said body, said body having a specific resistance exceeding 10 ohms/cm. at said given frequency, said body having a given preferential direction of Bloch partitions produced by internal anisotropy of shape substantially coincident with the longitudinal axis of said body and forming an angle with the magnetic field vector of the wave, and means to premagnetize said body below a field strength of the order of magnitude of the saturation field strength of the ferromagnetic material. 7

4. A device for amplitude modulating a cyclically recurring wave, comprising a, tubular body of substantially non-conductive, ferromagnetic material having a given axis means to generate a Wave of given frequency higher than the natural gyromagnetic resonance frequency of said ferromagnetic material means to couple said wave generating means to said body, said body having a specific resistance exceeding 10 ohms/cm. at said given frequency and a given preferential direction of Bloch partitions produced by internal anisotropy of shape said preferential direction being substantially coincident with said axis of said body and forming an angle with the magnetic field vector of the wave, and means to premagnetize said body below a field strength of the order of magnitude of the saturation field strength of the ferromagnetic material.

5. A device for amplitude modulating a cyclically recurring wave, comprising a substantially cylindrical body of substantially non-conductive, ferromagnetic material, means to generate a wave of given frequency higher than the natural gyromagnetic resonance of said ferromagnetic material, means to couple said wave generating means to said substantially cylindrical body, said body having a specific resistance exceeding 10 ohms/cm. at said given frequency, said body having a given preferential direction of Bloch partitions produced by internal anisotropy of shape substantially coincident with the axis of said body and forming an angle with the magnetic field vector of the wave, and means to premagnetize said body along said axis of the body below a field strength of the order of magnitude of the saturation field strength of the ferromagnetic material.

6. A device for amplitude modulating a cyclically recurring wave, comprising a substantially rod-like body of substantially non-conductive, ferromagnetic material, means to generate a Wave of given frequency higher than the natural gyromagnetic resonance frequency of said ferromagnetic material, a resonant cavity excited by said wave generating means, means to couple said body to said resonant cavity said body having a specific resistance exceeding 10 ohms/cm. at said frequency and having a given preferential direction of Bloch partitions produced by internal anistropy of shape substantially coincident with the axis of said body and forming an angle with the magnetic field vector of the wave, and means to premagnetize said body along the axis thereof below a field strength of the order of magnitude of the saturation field strength of the ferromagnetic material.

WILLEM JACOBUS VAN DE LINDT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,174,701 Koch Oct. 3, 1939 2,217,280 Koch Oct. 8, 1940 2,560,859 Gutton et al July 17, 1951 

